A liquid crystal display apparatus, employing active devices, such as thin-film transistors, features a thin thickness and a light weight, and is exploited as a high picture quality flat panel display. With the liquid crystal display apparatus, it is being contemplated to simplify the manufacturing process for realization of low production cost.
The liquid crystal display apparatus is being used as a display unit for a mobile terminal. The liquid crystal display apparatus used currently is predominantly a reflection type or semi-transmitting type liquid crystal display apparatus.
Of these, the reflection type liquid crystal display apparatus is designed so that a reflector provided within the apparatus reflects the light incident from outside. Reflected light is used as a display light source to render unnecessary backlight as a light source (see FIG. 3). As a result, the reflection type liquid crystal display apparatus lends itself more efficaciously to low power consumption, thinner thickness and lighter weight than the transmission type liquid crystal display apparatus.
On the other hand, the semi-transmission type liquid crystal display apparatus is designed to make efficacious display even on the occasion of the weak extraneous light. To this end, reflection display is made by exploiting reflected light, obtained on reflection of extraneous light by a reflector, at the same time as transmission display is made by exploiting backlight transmitted through an aperture (see 4c in FIG. 2) provided in a portion of the reflector.
The basic structure of a liquid crystal display apparatus is made up by a liquid crystal of the TN (twisted nematic) system, a monolithic polarization plate system, an STN (super-twisted nematic) system, a GH (guest-host) system, a PDLC (polymer dispersion) system or a cholesteric system, a switching device, a reflector provided inside or outside the liquid crystal cell, and a transparent electrode.
A reflector, which is used as a pixel electrode, contributes to enhancement of the intensity of light scattered in an orientation perpendicular to a display picture surface with respect to the incident light entered with a large incident angle. With this reflector, a brighter display may be produced. By covering a passivation film, having a roughness formed on a surface, with a reflection film, to provide a reflection surface with a roughness, an optimum reflection characteristic may be achieved. As for a typical conventional method for forming a rough surface on a reflector, see for example the JP Patent Kokai JP-A-10-319422.
A transparent electrode of ITO (indium tin oxide), used as a pixel electrode, may be formed as an uppermost layer, by layer separation from a drain layer, in order to improve the definition and aperture ratio of a display picture in a semi-transmission type liquid crystal display apparatus. The transparent electrode may also be formed on a surface of the reflector in order to prevent hillocks from being produced on the surface of the reflector formed of metal including Al.
The transparent electrode may also be used in a opening provided in the reflector in the semi-transmission type liquid crystal display apparatus, adapted for transmitting the backlight therethrough, so that the transparent electrode will operate as pixel electrodes. The transparent electrode is also sometimes used as wiring for electrically interconnecting a drain bus line and a drain electrode isolated therefrom.
In general, an active matrix substrate of a TN system is made up by a scanning line (termed a gate wiring or gate bus line) and a data line (termed a drain wiring or drain bus line), extending with orthogonal angles to each other, a reflector or a transparent electrode, formed in an area defined by these wirings, and thin-film transistors (TFTs) provided in the vicinity of the intersections of the gate and drain wirings. The active matrix substrate also includes a channel protection film on the TFT surface for guaranteeing the performance. On the TFTs, a reflector or a transparent electrode of this active matrix substrate, there is formed an orientation film for orienting the liquid crystal in a preset direction. The liquid crystal is sealed in a space defined by the active matrix substrate and by a counter substrate carrying another orientation film, a common electrode (transparent electrode) and a color filter, to complete the liquid crystal display apparatus.
By way of illustrating a conventional method for producing the liquid crystal display apparatus, the invention described in the JP Patent Kokai JP-A-10-319422 is now explained. FIGS. 81a-81d and FIGS. 82e-82g are cross-sectional views for schematically illustrating the processes for producing an active matrix substrate for a certain conventional reflection type liquid crystal display apparatus.
Referring first to FIG. 81a, in preparing an active matrix substrate, a gate electrode layer of, for example, Cr, is deposited on a transparent insulating substrate 101. Then, using a first mask, an exposed part of Cr is etched to form a gate bus line 102 and a gate electrode 103.
A gate insulating film 105, formed of SiNx, an a-Si(amorphous silicon) film 107a and a n+ type a-Si film 107b, which is to become an ohmic contact layer, are deposited in this order, as shown in FIG. 81b. Then, using a second mask, unneeded portions of the a-Si film 107a and the n+ type a-Si film 107b are selectively etched to form an island.
Then, referring to FIG. 81c, source/drain electrode layer of, for example, Cr, is then deposited and, using a third mask, unneeded portions of the source/drain electrode layer are selectively etched, for forming an opening in a channel area of the a-Si film 107a and a preset wiring pattern, to form a source electrode 108, which is to become a signal line, a drain electrode 109 and a drain bus line 110.
Then, using the source electrode 108 and the drain electrode 109 as an etching mask, the n+ type a-Si film 107b is etched to form an ohmic contact layer.
Then, referring to FIG. 81d, an organic insulating film 112 of, for example, polyimide, is deposited on the entire substrate surface. Then, using a fourth mask, a contact hole 113 for exposing a portion of the source electrode 108 and a contact hole 117c for exposing a portion of the gate bus line 102 are formed.
Then, referring to FIG. 82e, a portion of the organic insulating film 112 is selectively removed, using a fifth mask, and a rough portion 112b is formed, by heat treatment.
Referring to FIG. 82f, a layer of metal, such as Al, is deposited on the entire substrate surface and, using a sixth photo mask, the metal layer is selectively etched to form a reflector 104b. This forms a reflector having a rough surface. The reflector 104b is electrically connected to the source electrode 108 through the contact hole 113.
Finally, a transparent electrode layer of, for example, ITO (indium tin oxide) is deposited on the entire surface of the transparent insulating substrate 101, as shown in 82g. Then, using a seventh photomask, a preset portion of the transparent electrode layer is removed to form a drain terminal 117d connecting to the drain bus line 102 through the contact hole 117c and a hillock inhibiting film 152, adapted for inhibiting generation of hillocks in the reflector 104 to complete the manufacture of the active matrix substrate. The hillock inhibiting film may also be a transparent electrode.
In the above-described active matrix substrate, the reflector which is to become the pixel electrode and the transparent electrode are not provided on the same layer as the source/drain electrode layer, but are insulated and separated from each other by an organic insulating film. Consequently, for insulation and separation of the transparent electrode layer and the drain electrode layer, these do not have to be separated from each other in the transverse direction relative to the normal line direction of the active matrix substrate, so that these can be extremely close to or overlapped with each other. As a result, the black matrix for shielding the uncontrolled backlight light rays, leaking from an interstice produced when the transparent electrode layer and the drain electrode layer are separated from each other in the transverse direction, can be rendered small and hence the aperture ratio can be meritoriously increased. For this reason, the transparent electrode layer and the drain electrode layer are insulated and separated from each other by an organic insulating film.